1. Field of the Invention
This invention relates generally to polyimides. It relates particularly to linear aromatic polyamic acids and the corresponding linear aromatic polyimides which are prepared from the novel monomer 3,3'bis(3,4-dicarboxyphenoxy)diphenylmethane dianhydride.
2. Description of the Related Art
Linear aromatic polyimides have long been known for their usefulness in meeting aerospace requirements as high performance coating and structural materials. The label of "high performance" for these polymers has implied toughness, flexibility, low density, high temperature stability, chemical and radiation resistance and mechanical strength. Within the past few years, the list has expanded to include low dielectric properties, optical transparency, and improved solubility.
Polyimides have found wide use for insulating aerospace cables and wires, and they are finding new uses in the microelectronics industry. A polymer must be an excellent insulator (low dielectric constant) in order to service this industry. The lower the dielectric constant, the more efficient the circuit. The dielectric constant of commercially available, state-of-the-art polyimide film presently used for electronic applications ranges from 3.2 to 4.0 at 10 GHz depending upon moisture content.
The need exists also for high-temperature, highly optically transparent or colorless film and coating materials for application on large space components such as solar cells, space mirrors, thermal control coating systems, antennae, among many others. Commercial polyimide film is known for its bright yellow color and is not efficient in its transmission of solar energy.
State-of-the-art linear aromatic polyimides are praised for their thermooxidative stability and are by nature insoluble in common organic solvents. This thermal stability often translates into difficult engineering processability. Difficulty in processing is even experienced with the soluble polyimide precursor polyamic acid which during thermal imidization eliminates water and requires higher cure temperatures to remove the strongly associated amide solvent. These volatiles present little trouble for thin films but produce catastrophic voids in thick films and composite parts.
Accordingly, it is an object of the present invention to provide a linear aromatic polyimide which has a dielectric constant in the range 2.7 to 3.1 at 10 GHz.
Another object of the present invention is to provide a linear aromatic polyimide which is thermally stable and optically transparent in the visible region of the electromagnetic spectrum.
Another object of the present invention is to provide a linear aromatic polyimide which is soluble and can be prepared from the corresponding linear aromatic polyamic acid by chemical imidization to produce a polyimide powder.
Another object of the present invention is to provide a dianhydride monomer composition which is employed in the preparation of these polyimides.
Another object of the present invention is to provide a linear aromatic polyamic acid which can be chemically imidized to produce a soluble, thermally stable, highly optically transparent, low dielectric polyimide.